GPU Tessellation

Report
Damon Rocco
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Tessellation: The filling of a plane with
polygons such that there is no overlap or gap.
In computer graphics objects are rendered as
a triangular mesh
◦ A triangle is always a plane
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Thus, the triangles in a mesh can be
tessellated to increase detail
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In software tessellation provides an
interesting way of enhancing detail
In hardware tessellation allows a simple mesh
to be sent down to the GPU, converted to a
complex mesh, and then displayed
◦ Decrease memory to the card
◦ Increase rendering performance by decreasing the
number of polygons through the full pipeline
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With programmable tessellation what objects
get tessellated can be decided by the
programmer
This allows for objects closer to the screen to
be tessellated while objects far away are not
◦ Detail actually increases as objects get closer
instead of just getting bigger
◦ Resources aren’t wasted on meshes that are too far
away for tessellation effects to be viewed
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Xbox 360: ATI Xenos graphics chipset
◦ The first geometry tessellation unit
◦ “Programmable”
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ATI Radeon HD 5xxx series
◦ The first DX11 card
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NVIDIA “Fermi”
◦ NVIDIA’s first DX11 card
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DirectX 9: Dynamic Terrain Rendering on GPU
Using Real-Time Tessellation. Natalya
Tatarchuk. ATI Research published in
ShaderX7
DirectX 10: Instanced Tessellation in
DirectX10. Andrei Tatarinov. GDC ‘08
DirectX 11: New pipeline features three
tessellation stages, 2 are programmable
Natalya Tatarchuk
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Goal: Create a software library that would
allow tessellation exclusively on ATI cards.
Product: ATI GPU Tessellation library. An ATI
only library that worked in conjunction with
DirectX 9 (also 10.x) and was capable of
tessellating meshes
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A flat 900 polygon grid becomes a 1,000,000
polygon mountain-scape at inter-actable
rates
Andrei Tatarinov
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In the past complex shading models were
used to hide lack of detail in a polygonal
mesh
They can only do so much…
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The solution is to use on-the-card
tessellation to increase the physical detail in
the meshes
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A highly detailed mesh can be sent to the card
as a simple mesh and a displacement map
Trades ALU operations for memory bandwidth
ALUs scale faster than bandwidth
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Tessellation can also be used by Animators to
make their job easier
◦ Animate a low polygon mesh
◦ Tessellate and get detail for free
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New “patch” primitive defined by a set of
control points
Operation called refinement generates
triangle from each patch
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Per-patch operations
◦ Level of Detail computation
◦ Transfer of Basis
 Bezier -> B-spline
 B-spline -> NURBS
 Etc.
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Generating topology
◦ Generates a set of (u,v)-points
in the tessellation domain
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Programmable hardware tessellation
◦ 3 stages
 2 programmable shaders
 1 fixed function configurable tessellator
◦ New primitive “patch”
 Curved surface
 Easily converted to triangles
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Programmable tessellation can be imitated
using DirectX 10 features:
◦ Geometry Shader and Instancing 2.0
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Geometry Shader cannot do tessellation itself
◦ Outputs triangles serially
◦ Maximum output size of 1024 scalars
 16x16 grid of float4s
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Instead we can save small pre-tessellated
patches as instances
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The pre-tessellated patches represent the
results of tessellating every input patch
This data is combined in the vertex shader
to produce the desired effects
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Vertex shader inputs are too small to handle
an entire mesh
◦ Must be bound to shared buffers
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The tessellation mesh instances are of
varying quality, a LoD factor is used to
determine which mesh to select
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Tessellation factors can change across a
mesh as each patch edge has its own
tessFactor.
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Per-patch tessellation factors requires
multiple draw calls (it won’t in DX11)
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Since all of the meshes are stored in vertex
buffers the only inputs are the primitive id
and the vertex id
U = Vertex ID mod LoD
V = Vertex ID div LoD
LoD is based on vertex position
Mesh: 6118 patches, 256 vertices each, 8800 GT
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Tessellation presents a means of significantly
increasing detail without a performance cost
Tessellation is possible with DirectX 10
Some of the hiccups with the DirectX 10
implementation will be fixed by future
hardware implementations
Kevin Gee
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D3D11 HW Feature
◦ Required for DirectX 11 compatibility
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D3D11 Only
◦ No direct backwards compatibility
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Fundamental primitive is “patch”
◦ Not triangles
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Superset of Xbox 360 tessellation
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3 new stages added for Tessellation
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Programmable
Takes patch control points as an input
2 Output paths:
◦ Output of basis converted
control points to Domain Shader
◦ Output of the control points, a
tessellation factor, and
tessellation modes to the
Tessallator
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Tessellator operates on a per patch basis
Again 2 output paths:
◦ Outputs U,V points to Domain Shader for further
shading
◦ Outputs topology for primitive assembly
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Either receives a set of points from the Hull
Shader or the Tessellator.
The Domain Shader is invoked once per point
It outputs verticies
Images courtesy of ATI
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Creates a new authoring pipeline
◦ 1-pass process from input to optimally tessellated mesh
◦ Makes both animation and real-time applications faster
◦ Allows for a higher level of detail
Images courtesy of NVIDIA Gamefest 2008
Ignacio Castano
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Tessellation lets us send down low polygon
meshes to save memory and bandwidth which
are the main bottlenecks
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Tessellation allows for view dependent levels
of detail so the GPU doesn’t waste time
rendering unseen triangles.
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Subdivision Surfaces are a well explored
technique for increasing the detail of a mesh.
Previous GPU implementations required
multiple GPU passes
The new DirectX 11 tessellation hardware
allows us to do subdivision surfaces in a
single pass
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The hull shader is used for control point
evaluation.
Input is a face and its neighboorhood
Output is a regular bicubic bezier patch of
the face
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The Domain Shader evaluates the bicubic
Bezier patches and corresponding tangents
Reorders face patches for consistent
adjacency
Requires 69 instructions
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TATARCHUK N.: Dynamic Terrain Rendering on
GPU Using Real-Time Tessellation. ShaderX7
(Dec. 2008).
Tatarinov, A.: Instanced Tessellation in
DirectX10. GDC 2008. February 2008.
Gee, K.: Introduction to the Direct3D 11
Graphics Pipeline. Nvision 2008.
Castano, I.: Tessellation of Displaced
Subdivision Surfaces in DX11. Gamefest 2008

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